1. Field of the Invention
The present invention relates to a switch for closing a discharge path of a storage capacitor of a rectified power supply. It more specifically applies to the implementation of a switch meant to be controlled by a two-state signal that fluctuates between a reference potential, e.g., ground, and a high state (which has the same sign as the rectified power supply).
However, although the present invention is described hereafter in the context of this particular application, it should be noted that the present invention also provides a switch for selecting a voltage from between an a.c. voltage and a d.c. voltage (using the ground as a reference), which is meant to be controlled by a two-state signal also using the ground as a reference and having a high state that is the same sign as the d.c. voltage.
2. Discussion of the Related Art
FIG. 1 shows an example of a rectifier associated with a storage capacitor, for example, for a power converter such as a switch-mode supply system.
The rectifier generally includes a diode bridge 1 connected to an a.c. power supply (terminals E1, E2), for example, the mains a.c. voltage. A storage capacitor C is positioned between output terminals S1 and S2, respectively, of the rectifier to provide a rectified d.c. supply Vout. In the example shown in FIG. 1, capacitor C is associated with separate charge and discharge paths. The charge path is comprised of a diode D connected between a first terminal of the capacitor and terminal S2 (generally the ground). The discharge path is comprised of a thyristor Th mounted between the first terminal of the capacitor and the ground. Thyristor Th is a thyristor with its anode gate connected to a circuit 2 issuing a two-state control signal, using the ground as a reference.
FIG. 2 illustrates the operation of the rectifier as shown in FIG. 1. This drawing shows, in the form of timing diagrams, the course of voltage Vout according to a control signal Vg of thyristor Th, the on-periods of thyristor Th being illustrated in a third timing diagram.
It is assumed that at a time t0, capacitor C is charged and the voltage Vin issued by the rectifier is at its maximum Vm. Voltage Vout is also at its maximum Vm and thyristor Th is blocked. Voltage Vin starts to decrease in the second half of the halfwave and, at a time t1, block 2 sends a positive pulse (Vcc) for controlling thyristor Th. At this time, the thyristor turns on and voltage Vout abruptly jumps to the maximum charge level of capacitor C corresponding, for example, to the maximum amplitude Vm of voltage Vin. Assuming that a load is connected to terminals S1 and S2, capacitor C discharges until voltage Vin becomes once again higher than the voltage across the capacitor (time t2), that is, during the following halfwave. At that time, the thyristor blocks and capacitor C recharges via diode D1. This operation is repeated at each halfwave of the rectified a.c. voltage.
In some applications, the abrupt voltage jump at time t1 (i.e., at the turning-on of thyristor Th) raises problems. Such problems arise, for example, if voltage Vout is meant for a circuit synchronized on the mains, for example, a system for improving the power factor in a television receiver where signal Vout is used as a basis for the horizontal scanning of the screen.
In this type of application, the turning-on of thyristor Th might be seen on the image, the abrupt voltage jump creating interference at the mains frequency.
More generally, this problem appears in the switching between two voltage sources, i.e. a.c. and d.c sources.